Updating gateway attributes

[iychoi]

When updating gateway attributes, we use syndicate update_gateway command. However, the command updates attributes other than specified with default values.
For example, following command updates archive attribute to True but also updates allow_anon to False which is default.

syndicate update_volume name=pov archive=True

If the attribute allow_anon was True, this will change the attribute's value silently.
To prevent this, users had to specify other attributes with current values.

syndicate update_volume name=pov archive=True allow_anon=True private=False